With increase of needs for a high-speed access service, FTTH (Fiber To The Home) has spread worldwide. Most of FTTH service is provided by an economically excellent PON (Passive Optical Network) system in which one storage station side device (OSU: Optical Subscriber Unit) stores a plurality of subscriber side devices (ONU: Optical Network Unit) by time division multiplexing (TDM). In TDM-PON, as shown in FIG. 1, a burst transmitter in each ONU 200 transmits signal light within a transmission permission time notified based on dynamic bandwidth allocation calculation in an OSU 51, and a burst receiver in the OSU 51 receives signal light obtained by multiplexer, on a time axis, signal lights having different intensities and phases, which have been sent from the ONUs 200. Although the current leading system is GE-PON (Gigabit Ethernet™ PON) and G-PON (Gigabit-capable PON) whose transmission rate is at the gigabit level, a video distribution service has been progressed, and, in addition, applications used to upload/download a bulk file has appeared, thereby the capacity of the PON system is required to be further increased. However, in the TDM-PON, since a system bandwidth is expanded by increasing a line rate, significant deterioration of reception characteristics due to influences of the speeding up and wavelength dispersion and economic efficiency of the burst transmitter and receiver become problematic, thereby it is difficult to increase the capacity to more than 10 gigabytes.
To increase the capacity to more than 10 gigabytes, application of a wavelength division multiplexing (WDM) technique has been considered. FIG. 2 shows an example of WDM/TDM-PON in which the WDM technique is combined with TDM-PON. A downstream wavelength and an upstream wavelength are allocated to each ONU 200a, and temporal overlap of signals between the ONUs 200a is permitted until reaching the number M (M is an integer not less than 2) of the OSUs 51 in a parent node 100a. Thus, an increase of the number of the OSUs 51 can expand the system bandwidth without increasing the line rate per one wavelength.
The ONUs 200a to which the same upstream wavelength has been allocated are connected logically to the same OSU 51 and share a bandwidth. When the wavelength allocated to each of the ONUs 200a is fixed, the logic connection between each of the ONUs 200a and the OSU 51 is unchanged, and the ONUs 200a connected to the different OSUs 51 cannot share a bandwidth, thereby bandwidth fairness is not secured.
Meanwhile, Non Patent Literature 1 proposes wavelength-tunable WDM/TDM-PON in which an ONU has a wavelength-tunable function, as shown in FIG. 3. In this method, an OSU logically connected in the unit of ONU is changed by changing the wavelength allocated to the ONU, and all the ONUs can share a system bandwidth. Thus, a wavelength-tunable burst transmitter in each of the ONUs transmits signal light within a notified transmission permission time, using a transmission wavelength notified based on dynamic allocation calculation in the OSU, thereby the bandwidth fairness can be secured among all the ONUs.